When it counted. Marciano. Rahman for all his obvious physical power and size, struggled to get snap and weight on his punches most of the time, he couldn't even dent notorious glass chins like Oleg x2 and Barrett. It took Lewis laughing and giving him the perfect angle to land the big one.
Who cares what it took ?! Point is, his best punch carries more power. The ability to land it is another issue alltogether.
Rahman probably dealt more power in a single punch. But Marciano hit nearly equally as hard in both fists and could carry his power late into a fight.